A spark plug, by being mounted in a combustion device such as an internal combustion engine (an engine), is used for igniting a mixture in a combustion chamber. Also, the spark plug includes an insulator having an axial hole, a center electrode inserted in a forward end portion of the axial hole, a metal shell provided on the outer periphery of the insulator, and a ground electrode, joined to a forward end portion of the metal shell, forming a spark discharge gap between itself and the center electrode (for example, refer to JP-A-2008-108478).
In addition, the insulator includes an insulator nose length portion, formed in a forward end portion of the insulator, exposed inside the combustion chamber, and a tapered portion, extending from the rear end of the insulator nose length portion toward a rear end side, the outside diameter of which is increased toward the rear end side. Further, the insulator is retained on the metal shell by the tapered portion being directly or indirectly retained by a shoulder portion formed on the inner periphery of the metal shell so as to protrude therefrom.
Furthermore, in recent years, a reduction in size (diameter) of the spark plug has been demanded, along with which a reduction in diameter of the insulator has been required. The wall thickness of this kind of insulator reduced in diameter is made comparatively small.
Also, a highly efficient engine which has achieved downsizing, higher supercharging and compression, or the like, is proposed in order to respond to the tighter environmental regulations. In this kind of engine, when it operates, a very large vibration is applied to a spark plug, and the spark plug is heated to a higher temperature. In addition, with this kind of highly efficient engine, an insulator crack in a boundary portion between the insulator nose length portion and tapered portion is likely to occur for the following reasons.
That is, when a shock is applied to the spark plug along with an operation (vibration) of the internal combustion engine, stress is applied particularly to a region of the insulator in which there is a sharp change in outside diameter. Because of this, stress is applied concentrically to the boundary portion between the insulator nose length portion and tapered portion in which there is a sharp change in outside diameter. Herein, in the highly efficient engine, the insulator is more likely to be overheated, and stress applied to the insulator is also high. Consequently, as the boundary portion, by being overheated, is likely to take on a condition in which the mechanical strength thereof is decreased, an insulator crack is likely to occur in the boundary due to a high stress being applied to the boundary portion in this condition.
Furthermore, the heat of the insulator, by being transmitted from the tapered portion to the shoulder portion of the metal shell, is dissipated to the engine side. Because of this, the tapered portion and a region positioned close thereto are more easily rapidly cooled. Meanwhile, in the highly efficient engine, the insulator is heated to a higher temperature, as heretofore described. Because of this, a large thermal shock is applied to the tapered portion and the region positioned close thereto. As a result of this, there is concern that, due to a large thermal shock being applied thereto, an insulator crack occurs in the boundary portion between the insulator nose length portion and tapered portion, positioned close to the tapered portion, which is comparatively thin walled (comparatively low in mechanical strength).
Further, an insulator crack in the heretofore described kind of boundary portion is of particular concern in a spark plug which is reduced in diameter and whose insulator is comparatively thin walled.